1. Field of the Invention
The present invention relates to surface-coated metal nanoparticles, a method for producing the same, and a metal nanoparticle paste comprising the same. More specifically, the present invention relates to surface-coated metal nanoparticles comprising an organic coating film provided on a surface of each of the metal nanoparticles, a method for producing the same, and a metal nanoparticle paste comprising the same.
2. Related Background Art
Conventionally, Sn—Pb-based solder has been used for bonding electrodes of semiconductor devices and the like. However, from the viewpoint of environmental protection, novel bonding materials such as lead-free solder have recently been demanded. In addition, miniaturization and densification of semiconductor devices have required fine-pitch wiring formation, for which novel wiring formation materials have been demanded. Moreover, in bonding technologies for semiconductor devices and technologies for forming fine-pitch wiring, materials enabling low-temperature bonding or low-temperature wiring formation have been demanded in order to reduce load on semiconductor devices.
Conventionally, nanoparticles of metals such as Ag, Cu, and Ni have attracted attention as the above-described materials for the fine-pitch wiring formation. On pages 28 to 29 of “CHO-BIRYUSHI GIJUTSU NYUMON (Introduction to Superfine Particle Technology)” (Document 1, Ichinose Noboru, Ozaki Yoshiharu, Kashu Seiichiro, published by Ohmsha, Ltd., in July, 1988), it is reported that when such metal nanoparticles have a particle diameter of 20 nm or less, the metal nanoparticles can be sintered at a temperature by far lower than the melting point thereof (sintering temperature: 200° C. or below). For this reason, the metal nanoparticles are expected to be applied to low-temperature bonding for semiconductor devices, low-temperature formation of fine-pitch wiring, and the like.
Various methods such as vapor phase methods and liquid phase methods have been proposed as methods for producing metal nanoparticles. From the viewpoint of allowing mass production with a simple facility at low costs, a method in which metal nanoparticles are formed by reducing a metal ion in a liquid phase has been investigated. However, since metal nanoparticles are extremely likely to aggregate, the metal nanoparticles in an unmodified state are difficult to sufficiently exert the low-temperature sintering characteristics as described above.
For this reason, the aggregation of the metal nanoparticles is generally suppressed by forming an organic coating film comprising a polymer compound such as polyvinylpyrrolidone or polyvinyl alcohol on a surface of each of the metal nanoparticles. However, since such a polymer compound has a relatively high pyrolysis temperature, the following problem is caused. Specifically, if the heating temperature during bonding or during wiring formation is low, the organic coating film components are not sufficiently pyrolyzed, and remain in a bonding portion or in wiring. As a result, bonding properties, thermal conductive properties, or electrical conductive properties are deteriorated. Moreover, since bonding and wiring formation are often conducted in an inert gas atmosphere to prevent oxidation of electrode materials, bonding materials, and wiring materials, the following problem is also caused. Specifically, since the above-described polymer compound is difficult to be pyrolyzed in an inert gas atmosphere, electrical conductivity of wiring and the like and bonding strength of semiconductor devices are deteriorated.
Moreover, International Publication No. WO 2004/012884 (Document 2) discloses a method in which metal nanoparticles having excellent dispersion stability are obtained by subjecting a metal salt to a heat treatment in the presence of an amine compound or in the presence of both an amine compound and a fatty acid. However, even when the metal nanoparticles are used, electrical conductivity of wiring and the like and bonding strength of semiconductor devices are still insufficient.